Motors are, in general, subject to eccentricities due to run-out of the shafts. The eccentricity must be prevented as hard as possible in spindle motors which drive a variety of discs. Since the rotating shafts, from which an eccentricity is detected, are shaped like a cylinder, a contact-type dial gauge, an electric micro-meter, or a non-contact type electrostatic capacitance displacement gauge, and a laser displacement gauge are generally used for measuring an eccentricity at sections, i.e. the rotating shafts, subjected to detection.
A specific method of measuring the eccentricity is disclosed in, e.g. Japanese Patent Non-examined Publication No. H05-22710. Besides a first peak sensor that senses a peak value of an output from a displacement gauge, a second peak sensor is provided, so that a difference between the two peak values sensed by these two sensors tells an eccentricity.
In the case of motors driving a polygon mirror, a section subjected to detection is the mirror formed of facets, namely, the mirror is shaped like a non-cylinder. In this case, measurement in a contact manner is not allowed, so that a totally different measuring method is required. The contact-type sensor cannot be used as a matter of fact. Use of a non-contact type capacitance displacement gauge requires placing the gauge near to the motor as close as several tens μm, so that the sensors thereof are subject to collision with an angular polygon mirror. As a result, it is difficult to measure an eccentricity with the non-contact type capacitance gauge. Use of a laser displacement gauge available on the market has a speed of response on the order of micro-seconds, so that measurable range can be at most several hundreds rotations per minute. Since the polygon mirror rotates several tens of thousand rotations per minute, it is impossible for the laser displacement gauge to measure the eccentricity.
Polygon mirror motors are used in laser-beam printers and full-color copiers, and directly related to the printing quality, so that the motor of high accuracy is required. A polygon mirror is directly connected to an output shaft of a brush-less DC motor and is rotated at a high speed such as several tens of thousand rotations per minute. A method of measuring a dynamic eccentricity of the motor rotating in such a high speed is disclosed in, e.g. Japanese Patent Non-examined Publication No. H02-204713.
FIG. 9 shows a structure of a conventional instrument for measuring an eccentricity of the polygon mirror motor. Polygon mirror 81 rigidly mounted to rotating shaft 80 of the motor spins at a high speed. Laser beam L1 emitted from first laser light beam source 82 enters into polygon mirror 81 at a certain angle, and the reflected laser beam L2 passes through cylindrical lens 87 and travels to position detector 83 which detects a position of the laser beam having undergone lens 87.
On the other hand, laser beam L3 emitted from second laser light beam source 84 passes through half mirror 85, and the passed laser beam L4 enters to polygon mirror 81. The reflected laser beam L5 enters to half mirror 85, and its reflected light beam L6 enters to trigger generator 86.
As shown in FIG. 9, when a reflecting surface of polygon mirror 81 and incident laser beam L4 form right angles, reflection beam L5 enters to half mirror 85, and its reflection light beam L6 enters to trigger generator 86. Thus when polygon mirror 81 becomes a status as shown in FIG. 9, trigger generator 86 generates trigger signals, and observation of output signals from position detector 83 at this time allows measuring an eccentricity.
However, when polygon mirror has a certain angle from the status shown in FIG. 9, laser beam L1 emitted from first light source 82 travels through the path of laser beams L5, L6, and enters to trigger generator 86 besides laser beam L3. As already discussed above, laser beam L3 is emitted from second light source 84 and passes through the same path as beams L5, L6 do, and enters to trigger generator 86. The output signals from generator 86 thus need to isolate false signals made by first light source 82 and extract true trigger signals made by second light source 84.